Jet aircraft commonly include an auxiliary power unit. An auxiliary power unit provides electric power to the aircraft primarily during times when the jet engines are powered down. For example, when passengers are boarding the aircraft or deplaning, it is common to operate the auxiliary power unit to power the aircraft's systems.
An auxiliary power unit is a jet engine that is connected to a generator rather than to a propulsion system. Like all jet engines, the auxiliary power unit requires a supply of air. Because the auxiliary power unit is mounted inside of the fuselage instead of in a nacelle, it is not exposed to the ambient air around the aircraft and therefore it requires a dedicated pathway to supply it with air and a dedicated pathway to vent its exhaust. The pathway that supplies the auxiliary power unit with air commonly includes an inlet duct that connects the auxiliary power unit to an opening in the fuselage. The opening is covered by door that moves between a closed position and one or more open positions. While the door is in the closed position, air is precluded from entering the inlet duct. The door is kept closed while the auxiliary power unit is powered down. When the door is in an open position, air is able to enter the inlet duct and reach the auxiliary power unit. The door to the inlet duct is commonly mounted on a top portion of the fuselage. In some instances, the door is positioned near the vertical stabilizer.
An auxiliary power unit generates a substantial amount of noise while operating. The primary inlet noise produced by the auxiliary power unit has a frequency that corresponds with number of blades used in its compressor multiplied by the rotational speed. The higher the frequency of the noise, the more directional the noise will be. The high frequency noise propagates up the inlet duct in a direction opposite to the direction of the airflow and continues out of the inlet into the ambient airspace. Additionally, the high frequency noise will be redirected by solid reflective surfaces that it encounters as it travels outwardly through the inlet duct. When this high frequency, highly directional noise encounters the door to the inlet duct, it will be redirected in the same manner as a billiard ball encountering the bumpers of a pool table. Any person located in the path of this noise redirected by the inlet duct door will hear a persistent, shrill screech.
The door of the inlet duct commonly opens in a direction that faces towards a forward portion of the aircraft. If the passenger door to the aircraft is located on the same side of the aircraft as the door to the inlet duct, then passengers boarding or deplaning the aircraft while the auxiliary power unit is operating may experience a noisy environment if the noise emanating from the inlet duct is reflected off of the underside of the door towards them. This may be annoying and may make it difficult to carry on a conversation. This is an undesirable condition, especially in the case of a business jet where passengers have an elevated expectation of quiet and no enclosed walkway to shield them from the noise on their approach to the aircraft.
Additionally, the auxiliary power unit can also be operated in flight. The opening of the air inlet door is metered depending on airspeed and/or altitude. Noise from the air inlet can be broadcast to the ground under certain fight conditions and add to the overall noise signature of the airborne aircraft.
To address these situations, aircraft manufacturers commonly line the walls of the inlet duct with sound absorbing material. However, this solution can require an oversized inlet duct to accommodate the volume consumed by the sound absorbing material. Another method to reduce noise from the inlet duct is to add sound absorbing splitter vanes that span the cross-section of the duct. However, these vanes add flow resistance to the inlet system and may require a larger duct cross-section so as not to over restrict the airflow entering the auxiliary power unit. Thus, these solutions may add substantial cost, complexity, and weight to the aircraft.
It is desirable to provide an improved apparatus for reducing the sound generated by operation of the auxiliary power unit and transmitted by the inlet duct. Furthermore, other desirable features and characteristics will become apparent from the subsequent summary and detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.